Small neighbor as possible life source to seed Earth-like planet, if needed.

Large Moon

Right distance. Stabilizes tilt.

The right tilt

Seasons not too severe.

Atmospheric properties

Maintenance of adequate temperature, composition and pressure for plants and animals.

Right kind of galaxy

Enough heavy elements. Not small, elliptical, or irregular.

Giant impacts

Few giant impacts. No global sterilizing impacts after an initial period.

Biological evolution

Successful evolutionary pathway to complex plants and animals.

Right position in galaxy

Not in center, edge or halo.

The right amount of carbon

Enough for life. Not enough for Runaway Greenhouse.

Evolution of oxygen

Invention of photosynthesis. Not too much or too little. Evolves at the right time.

Wild Cards

Snowball Earth. Cambrian explosion. Inertial interchange event.

Why Life Might Be Widespread in the Universe

The fact that this chain of life existed in the black cold of the deep sea and was utterly independent of sunlight— previously thought to be the font of all Earth's life—has startling ramifications. If life could flourish there, nurtured by a complex chemical process based on geothermal heat, then life could exist under similar conditions on planets far removed from the nurturing light of our parent star, the Sun.

—Robert Ballard, Explorations

Several miles beneath the warm, life- and light-filled surface regions of the world's oceans lies a much harsher environment, the deep sea floor. Vast regions have little oxygen. There is no light. Much of this sea floor is composed of nutrient-poor sand, mud, or slowly precipitated manganese nodules. Temperature is a fraction above the freezing point. At least 6000 pounds of water pressure crush each square inch of matter at even average ocean basin depths. Because of these factors, except for small populations of highly specialized creatures that depend for food on the slow rain of detritus from far above, most of the deep-ocean bottom is a biological desert, long thought to be virtually lifeless and monotonous terrain.

Yet one type of environment found on the bottoms of all of Earth's oceans is neither flat nor sparsely populated. Running in linear ridges extending for thousands of miles along the sea floors are chains of active volcanic vents called deep-sea rifts. These rifts, which are situated along the margins of the great oceanic plates that make up the rocky base of the ocean floor, form undersea mountain chains. Here, in the great depth, darkness, and pressure of the sea, new crust is being created every hour, upwelling from below. These are places where the sea floor literally pulls away from itself, spreads, and in the process creates, in the endless, frigid night of the sea floor, the slow motion of tectonic plate movement known as continental drift. It seems the least hospitable environment on planet Earth. Ironically, it is teeming with life.

Amid constant earthquakes, hot magmatic lava wells up from subterranean regions in these rifts, where it encounters frigid sea water. Great gouts of this brimstone are instantly quenched as they meet the cool sea water, producing grotesque, pillow-like shapes as they turn to black rock. It is a place like no other on Earth, a region of unbelievable extremes where 2000°F lava meets 32°F water under a pressure of 400 atmospheres 2 miles beneath the sea. It is a zone of high-energy violence, where torrents of mineralized water flow like rivers out of the underworld, building great columns of metal precipitate from the hellish brew bubbling out of the Earth. Yet amid this deep-sea inferno, another and most curious phenomenon exists: submarine snow. Not the gentle snow that falls on land, but a blizzard of white material that flows out of the submarine fissures and then slowly settles onto the gnarled sea bottom. This "snow" is actually life, flocculated globs of microbes numbering in the billions and living amid the heat and poison spewing out of the vents. In utter darkness, unseen by any eye until a few humans probed the abyss in tiny, deep-diving submarines, life silently exists and thrives, creating this ethereal snowfall.